Particularly in the case of X-ray luminescent screens, the phosphors used are alkali metal halides, e.g. CsI, CsBr, NaI, RbBr and the like. Alkali metal halides are water-soluble and hygroscopic. They absorb and chemisorb moisture from the air. This undesirably leads to a reduction in the light yield and to decomposition of the phosphor layer.
To counteract this problem, there has been a tendency, according to the prior art, to protect the phosphor layer by means of a moisture-tight cover. In this context, reference is made, for example, to DE 42 22 946 C2, EP 0 932 053 A1 or EP 0 903 590 A1.
It is also known, in order to eliminate the above problem, to enclose the substrate which incorporates the phosphor layer in a housing under a vacuum. A housing of this type is expensive to produce. Over the course of time, the vacuum may be lost. For certain phosphor compositions, for example highly Eu-doped CsBr, discoloration which reduces the light yield occurs in vacuum.